Sales Tax Calculator for Small Businesses: Formulas, Nexus and What You Owe

Sales tax is calculated as the taxable price multiplied by the combined state and local rate, and if you only know the total you collected, divide by (1 + rate) to find the pretax sales. The harder question is not the arithmetic but whether you must collect tax at all and at which rate. The United States has no federal sales tax; each state, and many cities and counties, set their own rules, so the answer depends on what you sell, where you sell it and where your customer is.

The two formulas you need

Tax on a sale = taxable price x combined rate
Pretax sales from a tax-inclusive total = total collected / (1 + combined rate)
Tax collected from a tax-inclusive total = total collected - pretax sales

Worked example (illustrative rate)

A customer buys $1,250 of taxable goods in a location with a combined rate of 8.25%.

Tax = $1,250 x 0.0825 = $103.13
Total charged = $1,353.13

Now suppose your register recorded a tax-inclusive total of $5,412.50 for the day at the same rate.

Pretax sales = $5,412.50 / 1.0825 = $5,000
Sales tax collected = $5,412.50 - $5,000 = $412.50

A common mistake is multiplying the total by the rate ($5,412.50 x 0.0825 = $446.53), which overstates the tax because the total already includes it.

Is what you sell taxable?

Rules vary by state and product. In general:

Tangible goods are usually taxable, but many states exempt groceries, prescription drugs, or clothing under certain prices.
Services are taxed in some states and not in others; for example, states differ on repairs, cleaning, software support and digital services.
Digital products and software are taxed differently by state, and definitions are not uniform.
Resale and exempt sales: if a customer buys to resell or is an exempt organization, you generally need a valid exemption or resale certificate on file.
Shipping and handling may be taxable or not depending on state rules and how you charge for it.

Each state's department of revenue publishes a taxability guide. Because products are classified differently by state, check the state where the customer receives the goods.

Do you have to collect sales tax? Understanding nexus

Nexus is a connection to a state that creates a duty to register, collect and remit tax there.

Physical nexus: an office, store, warehouse, inventory (including inventory held by a fulfillment service), employees or sales representatives in the state.
Economic nexus: since the 2018 Supreme Court decision in South Dakota v. Wayfair, states may require out-of-state sellers to collect tax once their sales into the state exceed a threshold. Many states use $100,000 in annual sales into the state, and some also used a transaction count such as 200 transactions, though several states have dropped the transaction test. Thresholds and measurement periods vary, so check each state.
Marketplace facilitator laws: if you sell through a large marketplace, the platform often collects and remits sales tax for those sales in most states. You may still need to register for sales through your own site or for other obligations.

Not every state has a sales tax. Alaska, Delaware, Montana, New Hampshire and Oregon do not have a statewide sales tax, though Alaska allows local taxes and some other taxes may apply.

Origin-based vs. destination-based rates

Most states are destination-based: you charge the rate that applies where the customer receives the product. A few are origin-based, where you charge the rate of the seller's location for in-state sales. Combined rates can differ between neighboring addresses because of city, county and special district taxes. Use your state's rate lookup tool or tax software for the exact rate, and keep a record of how you determined it.

Step-by-step: set up sales tax

Determine where you have nexus for physical presence and for economic thresholds.
Register with the state department of revenue before collecting. Collecting without a permit is not allowed in many states.
Determine what is taxable among your products and services.
Set up your point of sale or e-commerce platform with the correct rates, or use sales tax software that updates rates automatically.
Keep exemption certificates for any customer who claims an exemption.
File returns monthly, quarterly or annually as your state assigns, even if you collected nothing. Late and non-filed returns bring penalties.
Hold the money apart. Sales tax you collect is not your money; it is held for the state. Keep it in a separate account so it is available at filing.

Example: a business selling in two states

A seller has a store in a state with a 7% combined rate and also ships online orders into a second state where it passed the economic nexus threshold. In a month, in-state taxable sales are $14,000 and shipped taxable sales to the second state are $9,000, taxed there at an illustrative 6.5% combined rate.

State 1 tax: $14,000 x 0.07 = $980
State 2 tax: $9,000 x 0.065 = $585
Total owed: $1,565, filed on two separate returns

The business must file in both states, and should not simply apply its home rate to every order.

Common mistakes

Charging tax but not registering, or registering but not filing zero-dollar returns.
Using one rate for every customer even though local rates differ.
Ignoring economic nexus until after a state letter arrives.
Spending collected sales tax as operating cash.
Forgetting exemption certificates and then owing tax on exempt sales in an audit.
Treating shipping as non-taxable without checking.
Not tracking sales by state, which makes thresholds hard to monitor.

If you discover you owed tax in the past

Many states offer voluntary disclosure programs that can limit penalties and the look-back period for sellers who come forward before being contacted. A CPA or sales tax professional can advise whether that fits your case.

When to consult a professional

Multi-state sellers, businesses selling digital goods or services and anyone who has crossed a nexus threshold should consider a CPA or a sales tax specialist. Software can help, but you remain responsible for correct filing.

Frequently asked questions

Do I charge sales tax on services?

It depends on the state and the service. Some states tax broad lists of services while others tax few. Check your state's department of revenue.

Is sales tax a business expense?

No. Sales tax collected is held on behalf of the state and is not revenue. Sales tax you pay on business purchases may be part of the cost of those items.

What if I only sell online?

You may still have nexus because of inventory, employees or economic thresholds. Review each state where you ship.

Can I get a resale certificate?

Most states issue a seller's permit or accept a multistate form for resale purchases. Check with your state.

Sources

Multistate Tax Commission — information on state tax policy and administration
U.S. Small Business Administration — links to state and local licensing and tax resources
Internal Revenue Service — federal tax information and a deduction for state and local taxes where applicable
Your state's department of revenue website — the final authority on rates, taxability and filing dates